How to Size a C&I BESS for Peak Shaving (Formula + Worked Example)
Key Takeaway
To size a C&I BESS for peak shaving, set power (kW) to the demand you must cut from the grid peak, and set energy (kWh) from how long that cut must last—then divide by usable SOC and discharge efficiency and add a reserve. Size from the worst peak days in interval data, not from motor nameplates or an average day.
Table of Contents
The RFQ That Quotes One “Battery Size”
Purchasers often send a one-line ask: “quote a 500 kWh C&I battery for peak shaving.” The OEM returns a cabinet whose PCS cannot follow a 200 kW spike, or a 2 MWh container that is energy-rich and power-poor for a 40-minute demand event. Demand charges and transformer ceilings care about kW at the meter for a defined duration. Energy is the area under that excess, not a marketing kWh sticker.
This article is a sizing method with a worked factory example. It is not a second comparison of use cases—that lives in peak shaving vs backup vs load shifting. Transformer and service limits that often set the demand target are covered in behind-the-meter transformer capacity limits. Cabinet vs container form factor after you have P and E is in containerized 5 MWh ESS vs cabinet ESS.
What You Must Decide (kW vs kWh)
| Decision | Meaning | Typical source |
|---|---|---|
| P_req (kW) | Max discharge to keep the site under the ceiling | Highest excess above target in the year |
| t_duration (h) | How long that excess lasts as a contiguous event | 15-minute (or better) interval data |
| E_event (kWh) | Energy delivered during the event | Integral of excess, or P × t if the peak is flat |
| Usable SOC / DoD window | Fraction of nameplate you may actually discharge | OEM datasheet (often ~80–90%; confirm) |
| Discharge path efficiency | PCS + battery path losses on the way out | OEM figure; do not assume 100% |
| Reserve | Forecast error, growth, second peak the same day | Often +10–20% energy or next power step |
Weltrus C&I ESS product discussion typically maps to cabinet and container bands in the 50 kW through 1 MWh, 2 MWh, and 5 MWh classes. Treat those as bands, not a substitute for a named SKU with rated P and E.
Core Formula
Peak-shaving sizing is two numbers: kW and kWh.
Power
P_req ≥ max(P_load(t) − P_target) over the study year (or the billing months that set demand charges).
- P_load(t) — interval demand at the billing meter (or the feeder you intend to control).
- P_target — the ceiling you will enforce (tariff ratchet, contracted demand, or transformer/service limit).
Use the worst peak days, not the annual average. If solar is on site, run the same math on residual load (load minus PV) if the BESS will see the net meter.
Energy for one peak event
Preferred: integrate max(P_load − P_target, 0) over each contiguous peak window; take the largest event (and check whether two events can occur before recharge).
Rectangular estimate (conservative when the excess is nearly flat):
E_event ≈ P_req × t_hours
Nameplate energy
E_nameplate ≥ E_event / (DoD_usable × η_discharge) × (1 + Reserve%)
Illustrative factors only—replace with the OEM sheet: usable DoD 0.90, one-way discharge factor 0.92, reserve 15%. Then map P_req and E_nameplate to a PCS and battery class. A 1 MWh battery that can only discharge at 50 kW will not shave a 220 kW spike.
Worked Example: Factory Peak
Illustrative light-industrial site (replace with your intervals):
| Input | Value |
|---|---|
| Interval data | 15-minute demand, 12 months |
| Observed annual peak | 820 kW |
| Target ceiling | 600 kW (transformer / tariff goal) |
| Longest contiguous excess above 600 kW | 1.5 hours, near-flat ~220 kW excess |
Power: 820 − 600 = 220 kW. Round to a 250 kW PCS class if the OEM steps that way (headroom and modest growth).
Event energy (rectangle): 220 kW × 1.5 h = 330 kWh.
Nameplate with factors 0.90 × 0.92 and 15% reserve: 330 / (0.90 × 0.92) × 1.15 ≈ 458 kWh.
Product mapping (bands, not SKUs): ~220–250 kW / ~0.5 MWh often sits in a cabinet class inside a 50 kW–1 MWh band. Longer second-shift peaks step toward 1–2 MWh. Campuses with several buildings may need containerized classes up to 5 MWh. Confirm PCS continuous and overload ratings against the 15-minute spike shape—not only energy.
If the same day has a morning and afternoon peak and the tariff resets monthly (not daily), check whether the battery can recharge between events at the allowed grid or solar rate. If it cannot, energy must cover both events or the second peak will reset the demand charge.
Do Not Mix Peak Shave With Backup Hours
Peak shaving is usually power-first and duration-limited. Backup is critical load × autonomy hours. Load shifting is a daily energy window and tariff structure. One cabinet can be programmed for more than one mode, but the controlling use case must set P and E on the RFQ. Do not hide eight hours of whole-site backup inside a peak-shave quote without stating it—power and energy both grow, and so does thermal and fire design.
Data Before RFQ
- Twelve months of interval demand (15 min or better); note DST and meter replacements.
- Tariff demand-charge structure, ratchets, and the months that set the peak.
- Why P_target exists (cost vs transformer/service capacity).
- Growth: new process line, EV chargers, HVAC.
- On-site PV and whether control is on net or gross load.
- Indoor/outdoor, ambient, noise, fire code, interconnection.
- Certification pack expected (for example IEC 62619, UN38.3, CE—as applicable to destination).
If you only have monthly kWh, you cannot size peak shaving honestly. Ask the utility or logger for interval files before OEM bidding.
Pitfalls That Break ROI
- Nameplate motors instead of coincident demand — installed kW is not the 15-minute peak.
- Treating nameplate kWh as deliverable — omit DoD and efficiency and you empty the stack before the peak ends.
- Under-sizing PCS power — extra kWh cannot clip a spike the inverter cannot follow.
- No EMS peak-shave mode — hardware without a ceiling, meter point, and recharge rule.
- Silent backup scope — mixed RFQ without priority.
- Ignoring recharge between dual peaks — second event sets the bill.
- Skipping thermal class — climate and noise change liquid vs air choice after P/E are known.
OEM Fields to Paste
| Field | Why it matters |
|---|---|
| Rated P and E | Match P_req and E_nameplate |
| Usable energy / SOC window | Convert nameplate to deliverable kWh |
| PCS continuous and overload | Follow the 15-minute spike |
| EMS peak-shave logic | Meter point, target, recharge, fail-safe |
| Thermal (liquid / air) | Ambient, noise, density |
| Fire / BMS scope | Pack vs system approach |
| Cert pack | IEC 62619, UN38.3, CE as required |
| Lead time / MOQ / export pack | Schedule, not only kWh price |
Weltrus supplies C&I energy storage hardware and related electrical products; send the interval peak summary, P_target, and longest event duration so the quote maps to a confirmed power/energy class rather than a single kWh line item.
Boundaries
- Not a stamped interconnection study or tariff legal opinion.
- Not a price or IRR for your site (demand rates vary by utility and year).
- Not the use-case comparison article (see related peak vs backup vs shift).
- Not permission to skip OEM usable-energy and PCS ratings.
If interval data shows a thin, brief spike, a smaller P_req with modest energy often beats a large kWh cabinet that cannot discharge fast enough. If the transformer is the ceiling, size the BESS to the thermal limit you actually intend to hold—then confirm protection and utility rules separately.
Send Peak kW, Duration, and Target Ceiling
Share 12-month interval highlights (or the file), P_target, and whether backup is in scope. Weltrus can map a C&I ESS class in the 50 kW–1 MWh–2 MWh–5 MWh discussion bands—or flag when the RFQ is still mixing use cases.
Frequently Asked Questions
How do I calculate the size of a C&I BESS for peak shaving?
Set required power as peak load minus your demand target. Set energy from how long that excess lasts (integrate interval data or use P × hours). Divide by usable SOC and discharge efficiency, add a reserve margin, then map to a cabinet or containerized C&I ESS class.
Is kW or kWh more important when sizing peak-shaving storage?
Both matter, but power (kW) is usually the first constraint. If the battery cannot discharge at the required kW, extra kWh will not shave the demand spike. Energy must still cover the full peak duration after usable SOC and efficiency losses.
What data do I need before requesting a C&I BESS quote for peak shaving?
Provide 12 months of interval demand (ideally 15-minute), your target demand ceiling, the longest contiguous duration above that ceiling, growth plans, on-site solar if any, site thermal and fire constraints, and required certifications such as IEC 62619, UN38.3, and CE.
Can one C&I BESS do peak shaving and backup at the same time?
Yes, but the controlling use case must set the size. Peak shaving is usually power-driven for short peaks; backup is driven by critical load and autonomy hours. State priorities in the RFQ so the OEM does not under-size power or under-size energy.
What C&I ESS size bands does Weltrus offer for peak shaving?
Weltrus supplies commercial and industrial energy storage covering 50 kW through 1 MWh, 2 MWh, and 5 MWh cabinet and containerized classes, with BMS, EMS, fire suppression, and thermal management. Map your P_req and E_nameplate to the nearest confirmed SKU—do not treat the band as a guaranteed SKU.
